Alwalton put-in

The tidal limit on the Nene lies far downstream, at the Dog-in-a-Doublet lock and sluice east of Peterborough, completed in 1937. The water at Alwalton is held entirely by the navigation's locks and weirs rather than by any tide: Alwalton Lock controls this reach, with a separate weir stream carrying the overflow past a floating exclusion barrier. The next lock downstream is at Orton, beyond Ferry Meadows and the Nene Park watersports centre on Overton and Gunwade Lakes; Water Newton Lock lies upstream towards Wansford.

The Great Oolite limestone that underlies this stretch of the valley forms the escarpment known as Alwalton Lynch, on the south bank at Alwalton. The same stone was quarried here as Alwalton Marble between the 12th and 13th centuries, with quarrying continuing into the 1920s, and was cut into building stone used in the cathedrals at Lincoln, Peterborough, Bury St Edmunds and Ely, and in Southwell Minster.

The opposite bank carries Castor Flood Meadows, a 41.8-hectare Site of Special Scientific Interest and one of the few remaining stretches of unimproved flood meadow on the river. It was notified in 1986 for its grassland flora, which includes slender tufted-sedge, early marsh-orchid and the nationally scarce narrow-leaved water-dropwort.